Based on discussion in #2031, this adds an abstract service for SVG badges. I started with Readthedocs and the other services can be done as a follow-on.
I called it **BaseSvgScrapingService** rather than **BaseSvgService** to clarify that it's for badges from svg source data – not svg badges, which is all the badges.
Since I don't expect the svg parsing function to be used anywhere else once the services are refactored, I moved it into the class. I added a default value for `valueMatcher`, which works on Shields-style badges and seems to be used more than once.
The tests are based on XmlBaseService. I added one for valueMatcher, and also moved the SVG parsing badge here. Testing on codacy + vso should ensure the old `fetchFromSvg` is still working.
We use arrow functions in most places; this enforces it.
Passing arrow functions to Mocha is discouraged: https://mochajs.org/#arrow-functions
This was a mix of autofixes and hand adjustments.
